There has hitherto been used an internal combustion engine provided with a thermal reactor, in which exhaust gas discharged from the engine is caused to undergo re-combustion with secondary air in order to purify the exhaust gas discharged from the engine of the motor vehicle. In an internal combustion engine equiped with such a reactor, it is intended to enrich the fuel-air mixture supplied to the engine to a certain degree, to thereby enhance the operation performance of the engine while purifying the exhaust gas finally discharged to the atmosphere in a low speed rotation region of the engine. However, the hitherto known apparatus of this kind does not exhibit the desired and expected functions to a satisfactory degree and has encountered difficulty in assuring a stable operation, particularly in the operating state in which the intake air quantity is reduced from the standpoint of exhaust gas purification. A typical internal combustion engine which is operated with an enriched fuel-air mixture in the low speed rotation region, is a rotary engine.
U.S. Pat. No. 3,827,237 discloses a fuel-air ratio control method according to which the quantity of secondary air supply is controlled as a function of temperature of the thermal reactor while the fuel supply quantity is controlled in dependence on the output signal from an oxygen sensor. However, because the secondary air supply is not controlled by the very oxygen sensor that serves for controlling the fuel supply quantity but in dependence on the output signal from the other sensor destined from detecting the temperature of the reactor, the control system of the prior art cannot maintain in the intake system of the engine the fuel-air ratio which assures an optimum operating condition with a reasonable accuracy.